The present invention generally relates to rotary machines, such as steam and gas turbines, and, more particularly, is concerned with a rotary machine having a seal assembly to control clearance between tips of rotating rotor blades or “buckets” and a stationary outer casing or stator of the rotary machine. Specifically the invention relates generally to an abradable seal member provided to the rotating portion of a turbine. The invention also relates to a turbine and/or turbine bucket including an abradable seal member, and a method for sealing an interface between rotating and stationary components of a turbine or other machine.
Steam and gas turbines are used, among other purposes, to power electric generators, and gas turbines also are used, among other purposes, to propel aircraft and ships. A steam turbine has a steam path which typically includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. A gas turbine has a gas path which typically includes, in serial-flow relationship, an air intake (or inlet), a compressor, a combustor, a turbine, and a gas outlet (or exhaust nozzle). Compressor and turbine sections include at least one circumferential row of rotating buckets. The free ends or tips of the rotating buckets are surrounded by a stator casing.
The efficiency of the turbine depends in part on the radial clearance or gap between the rotor bucket tips and the surrounding casing. If the clearance is too large, more of the steam or gas flow will leak through the gap between the rotor bucket tips and the surrounding casing, decreasing the turbine's efficiency. If the clearance is too small, the rotor bucket tips may strike the surrounding casing during certain turbine operating conditions.
It is known that the clearance changes during periods of acceleration or deceleration due to changing centrifugal force on the bucket tips, due to turbine rotor vibration, and due to relative thermal growth between the rotating rotor and stationary casing. During periods of differential centrifugal, rotor vibration, and thermal growth of the rotor and casing the clearance changes can result in severe rubbing of the moving bucket tips against the stationary casing. This increase in bucket tip clearance results in efficiency loss.
Clearance control devices, such as abradable seals, have been proposed in the past to accommodate rotor-to-casing clearance change. See, for example, U.S. Pat. Nos. 6,340,286 and 6,547,522, both assigned to General Electric Company. This protection feature allows the designer to decrease the cold built clearances of the turbine or engine, which decreases unwanted leakage, thus improving the performance and/or efficiency of the turbine or engine. The abradable seal or coating is typically applied to the stationary components, with the seal segments machined into and/or caulked into the rotor assembly.
However, these solutions are not believed to represent an optimum design for controlling such clearance. Consequently, a need still remains for an innovation which will provide a solution to the above-described clearance control problem without introducing any new problems in place thereof.